The invention relates to an electric capacitor formed of dielectric layers provided on at least one side with metal layers as coatings, and which are composed of partially crystallized and pre-shrunk polyethylene terephthalate. The coatings are alternately electrically conductively connected to metal layers and are applied with opposing polarity to end faces of the capacitor body, particularly by the Schoop process. The dielectric layers are stacked or shaped to form a flat-pressed winding.
The invention further relates to a method for production of a capacitor for use as a solderable chip component, wherein from dielectric layers metallized on at least one side and composed of partially crystallized polyethylene terephthalate, capacitor bodies are produced which are formed of a round winding, and which are then shaped in a manner known per se to form a flat-pressed winding. The bodies are simultaneously preshrunk, and metal layers are applied to the end faces for contacting the coatings arranged alternately on the dielectric layers.
The invention further relates to a method for production of a capacitor, in particular for use as a chip component, wherein strips of polyethylene terephthalate metallized on at least one side, and having an edge cut in a wave formation, are applied in layers to a drum so as to form a starting capacitor. The starting capacitor is divided into the desired individual capacitors at right angles to the layer planes. Capacitively non-active intermediate layers are arranged on a number of dielectric layers and coatings which form at least one parent capacitor. The capacitively active dielectric layers with the coatings for the next parent capacitor are arranged on the intermediate layers. The starting capacitor formed in this way is provided with end contact layers, is then tempered on the drum at approximately 150.degree. C., and is simultaneously compacted by preshrinkage. Only then is the starting capacitor divided into the desired individual capacitors in the region of the intermediate layers and in the direction at right angles thereto.
Capacitors in accordance with the present invention consist both of flat-pressed winding capacitors and stack or layer capacitors, as are known.
Whereas a number of publications are available for flat-pressed winding capacitors and the production thereof, thus obviating the need to give a precise reference to publications, stack or layer capacitors in accordance with the present invention, and the production thereof, are described, for example, in German Pat. No. 1 764 541 (corresponding to U.S. Pat. Nos. 3,670,378 and 3,728,765, incorporated herein by reference). These capacitors have previously been manufactured in very large numbers (up to or above one million per day) and are also commercially available.
Another form of stack or layer capacitors to which the present invention likewise relates, are described in German Patent Application No. P 33 42 329.6, incorporated herein by reference. In contrast to other conventional stack or layer capacitors, these capacitors are bifilar and are thus low in induction. A capacitor of this type is illustrated in the attached FIG. 3 and will be explained in the description of the drawings.
The known electric capacitors which have polyethylene terephthalate as dielectric have proven themselves extensively in practice on account of the particularly good dielectric and electric properties of this dielectric.
However, if such capacitors are to be attached as a chip component to the circuit boards of printed circuits, difficulties occur since the heat of the liquid solder, which reaches temperatures of a maximum of 260.degree. C., and which the capacitors indirectly or directly contact for a period of ten seconds, results in changes in the dielectric, such as for example considerable additional shrinkage, deformation of the capacitor body, breaking-off of the applied metal layers, and a change of the dielectric behavior (loss factor, dielectric constant), so that previously it was impossible to use such capacitors as chip components. In such cases it was necessary to use capacitors having a ceramic dielectric, or else to provide that capacitors having a synthetic dielectric were protected from the effects of heat in a special and structurally elaborate manner.
In the production of stack or layer capacitors on a drum, further problems occur since when a plurality of parent capacitors (rings) are wound one above another onto the drum, the winding pressure reduces from the inside towards the outside. Also, during the tempering of the capacitors for purposes of compacting, the innermost parent capacitors (rings) are subjected to an increased pressure produced by the shrinking process. As a result of the lower pressure in the compacted starting capacitor the outer parent capacitors exhibit lower capacitances--which in part are substantially lower--than for example the innermost parent capacitor. These deviations can amount to up to 20%. These difficulties occur mainly when very thin dielectric layers are used, for example, those having a thickness of 3 .mu.m and less. In order to compensate the lower capacitance of individual capacitors produced from outer parent capacitors, the previous practice has been to make such capacitors longer when they are separated from the parent capacitor.